If you no longer want a user to have access to your team in Metatask, you can disable this team member. To do this, open the account drop down menu and go to [Team name] Settings. 

You need the tab "Users". Click on the name of user you want to disable and go to the user details panel on the right. Then click the button "Disable user". 

If this user has some tasks assigned, Metatask will double-check with you about disabling her. Once you click "Disable user", this team member will be logged out of Metatask and no longer have access to her account in your team. 

If you need to enable the user back, press "Active" option and choose "Disabled" in the drop-down list. Then click on user name to access user details panel and press button "Enable user".